Thermionic peak voltmeter



Oct. 9, 1928. 1,687,234

1;. M TERRY THERMIONIC PEAK VOLTMETER Filed June 10, 1926 T 22 fig;

by My Patented @ct. ii), 3928.

warren v STATES rattan PATENT orFicE.

DONALD MERCER TERRY, OF NEW YORK, N. ASSIGNOR TO WESTERN ELECTRIC COM- IPANY, INCORPORATED, OF NEW YORK, N. Y A CORPORATION OF NEW YORK.

THER'MIONIC PEAK VOLTMETER.

Application filed June 10, 1926. Serial No; 114,971.

thermionic voltmeter disclosed in R. A.

Heising Patent 1,232,919, issued July 10, 1917. In accordance with that invention the input circuit of an electron discharge device is adjusted through a slide wire resistance to reduce the plate or output current to zero and the potential drop across the slide wire resistance is measured by means of a voltmeter. The voltage to be measured is then impressed onthe input circuit and the resistance again adjusted to reduce the output current to zero. Another measurement is then made of the voltage drop across the resistance and the difference of these voltage drops is computed to obtain the voltage to be measured. It will be seen that a direct indication of the peak voltage cannot be obtained by this method.

An object of'the present invention is to measure alpeak voltage between two points of an electrical networkwithout taking current from that network and without mtroducing frequency errors.

Another object of the invention is to d1- rectly and accurately indicate the value of the peak voltage to be measured.

In accordance with this invention, an electron discharge device is provided with separate sources of potential for adjusting the normal output current of the 'device to an optimum operating value and for reducing the space current to this operat ng value when a voltage to be measured is impressed upon the input circuit, so that the voltage can be directly .and accurately determined without calculation.

. Thisinvent-ion can be more readily understood by reference to the following detailed I description in connection with the drawing which shows diagrammatically one embodiment of the invention.

Referring to the drawing, the peak voltmeter comprises an electron discharge device 1 having input conductors 2 and 3, upon which is impressed the voltage it is desired to measure. This device consists ofan electron emitting cathode or filament 4, an anode 5, and a control electrode or grid 6, enclosed in an evacuated glass vessel. The cathode 4 is connected externally to a heating circuit including a battery 7, an adjustable resistance 8, and an ammeter 9. This ammeter indicates the current flowing in the filament and serves as a means for maintaining the filament at the most efficient operating temperature. An arm 10 of a gan switch 11 opens and closes the filament circuit. The grid 6 is connected to conductor 2 of the input circuit and conductor 3 is connected through switch 12, conductors 13 and 14, assuming all adjustable contacts are at zero, to one side of the filament 4, thus completmg the input circuit. The output circuit of the device comprises anode 5, battery 15 and milliammeter 18, which is connected tov the common point of the filament 4. A source of voltage 17, such as a battery, is provided in the input circuit to impress a negative potential on the grid 6 of the dis charge device 1. The movable contact 18 cooperating with fixed contacts 18 provides means for varying the voltage; and the resistances 19, in alternate leads of the contacts make it possible to obtain this change of voltage without opening the grid circuit or short ci'rcuiting the battery 17. This arrangement will be hereinafter designated as the coarse adjustment of the grid voltage. Another source ofvoltage 20 is connected in the input circuit through arm 21 of switch 11, and adjustable resistance or potentiometer 22, to provide a fine adjustment of the grid voltage. A voltmeter 23 is connectedacross these two sources of voltages to indicate the value of the voltage measured. A multiplier 24 comprising a variable switch and different values of resistances is provided in the voltmeter circuit to cover a large range of voltages which may be measured in accordance with this invention.

A battery 25 connected to anauxiliary potentiometer 26 through arm .27 of gang switch 11, is utilized to maintain the potential of the anode at a desired operating point on the characteristic curve of the discharge device as will be further explained hereinafter. The potential 25 is not indicated on the voltmeter 23 and is used solely to adjust the plate current under normal conditions, i. e. with no external source connected to input conductors 2 and 3. A resistance 28 and a switch 29 are connected across the input I the circuit, since, if the impedance of the circuit across which the peakvoltage is to be measured should be of as high an order of magnitude, the shunt resistance would afford an unnecessary load which would probably lower" the voltage to be measured.

The operation of the peak voltmeter is as follows:

The filament circuit is closed by the arm 10 of gang switch 11 and the filament current adjusted by the resistance 8, until the desired value is indicated on the ammeter 9. Assuming all the variable adjustments in the input circuit are at zero, a space current will be indicated on the milliammeter 16. WVith a suitable type of tube this space current may be of the order of 1.5 milliamperes. It has been found, that if the space current is reduced to a recommended zero operating point of 0.1 milliampere, a wide range of peak voltages can be measured With accuracy and sensitivit The plate current is adjusted and maintained at the recommended zero operating point of 0.1 milliampere by the battery 25 and the adjustable potentiometer 26, wh ch 1mpresses a negative potential on the grid sulficient to reduce the space current to the recommended zero operating point of 0.1 milhampere and change the indication on the milliammeter 16. WVhen this adjustment is completed, the alternating voltage source to be measured is connected to the conductors 2 and 3. The efiect of this voltage source is to correspondingly increase the plate current to a value higher than the optimum space current.

This increase is overcome and the plate current maintained at its normal operating value of 0.1 milliampere, by advancing the movable contacts of the coarse adjustment of potential 17 and the fine adjustment 22, to impress a negative potential on the grid. When the milliammeter 16 indicates that the plate current has been reduced to the value of 0.1 milliampere, the peak voltage is indicated directly by the voltmeter 23. Hence, no subtraction of voltages is necessary and the grid battery can be more accurately maintained; Further-- more, by the former subtraction methoda sufliciently sensitive voltmeter could not be used since it had to be capable of reading the voltage across the potentiometer 26 which is relatively small compared to the peak voltages which it may be desired to measure.

Because of the extremely high input impedance to direct current of an electron discharge device, this peak voltmeter lends itself admirably to the accurate measurement of weak direct currents, such as often occur acrosshigh resistance circuits; However, for

alternating current peak voltage measurements it is necessary that the peak voltmeter be calibrated, to correct the inherent error which results from the fact that the average value of plate current from rectified peaks must be less than the maximum current of the cutoff peaks when the device is operated above the zero point. of its characteristic curve. It'is desirable to operate the discharge device above the zero point, since in lowering the operating point, a point is soon reached, even with a very sensitive plate-current meter, where the inaccuracy of the grid potential adjustment can result in a large observational error. With the peak voltmeter of this in vention an ordinary voltage measuring instrument can be used and will be comparable in range with the magnitude of the voltages to be measured.

The invention is adaptable for'measurin'g a 35 large range of voltages bymeans of the multiplier 24. However, when measuring peaks of the order of 150 volts, additional batteries 30 may be connected in series with the battery 1 7, by the switch 12, to impress a higher nega trve potential on the grid of the discharge de- V106.

What is claimed is:

,1 A peak voltmeter comprising an electron discharge device having input and output circuits, a current indicating device in sa1d output circuit, terminals for impressing upon said input circuit a voltage to be meas ured, a source of potential in said input circuit for maintaining the normal current in 109 sa1d output circuit at a definite operating value, a second source of potential in said input circuit for reducing the currentin said output circuit to said operating value when the voltage to be measured is connected to said terminals, and means for directly indicating the value of the potential of said second source to give the Value of the voltage to be measured.

2. A peak voltmeter comprising an electron 110 discharge device having a cathode, a control electrode and an anode, input and output circuits for said device, a current indicating device and a source of potential in said output circuit, terminals for impressing on said input circuit a voltage to bemeasured, a variable source of potential in said input circuit for adjusting the normal operating current of said device to an optimum operating value,

a second variable source of potential for ad justing the output current ofsaid device to said operatingvalue when said terminals are connected to avoltage to be measured, and a measuring instrument for directly indicating the value of the potential of said second source to give directly the value of the voltage to be measured.

3. The method of determining the magnitude of a voltage of an alternating current which comprises applying steady potentials to the input and output circuits of a vacuum tube, impressing a negative potential on the control member of said tube to reduce the space current flowing in the output circuit to a low finite value, applying the alternating current voltage to the input circuit of said tube, impressing a second voltageon the control member of said tube to oppose the alternating current voltage, reducing the space current to said finite value, and measuring the magnitude of said second voltage.

In Witness whereof, I have hereunto subscribed my name this 3rd day of June A. D.,

DONALD MERCER TERRY. 

